1. Field of the Invention
This invention relates to the field of disk shaped information recording media, and in particular to the field of providing protective covers for such optical information disk-shaped recording media.
2. Description of the Prior Art
In optical disk information recording systems, the data is recorded on radiation sensitive media. The disk is ordinarily comprised of a disk-shaped substrate, usually aluminum, onto which a thin film of radiation sensitive media is deposited. Because of the optical nature of the surface of this disk, the surface ordinarily is in need of protection from dust and dirt contamination as well as for providing a means of defocusing such dust particles. In the prior art, this protection has been provided by a radiation-transmissive protective covering, which is fixedly secured to the disk surface above the media. The prior art has taught the use of both "thin" media covers (on the order of approximately 0.005 inches thick) and "thick" media cover (on the order of approximately 0.040 inches. Thick protective covers or windows have been attractive because of their ease of manufacture and relatively tighter mechanical tolerances, thus allowing for repeatable and accurate optical qualities.
In the prior art in which a relatively thick protective cover is used, the prior art has taught the bonding of inner and outer diameter spacers to the disk substrate and thereafter bonding the protective window to the inner and outer diameter spacers. However, because of the different materials used to form the protective cover and the disk substrate, the protective cover usually has a thermal expansion coefficient that is different from that of the support substrate. It has been found that if rigid adhesives are used to bond the protective cover the spacers and the substrate together, a temperature change of 20.degree. F. can result in a protective cover distortion of 5-10 mills, causing the protective cover to bow or warp relative to the disk surface. Such distortion can cause the aberration of the radiation beam, with resulting read and write errors, and can also degrade the device performance by effecting the dynamic stability of the rotating disk.
The prior art has addressed this problem by focusing on the proper selection of adhesives and spacer material in order to achieve a strong, but flexible bond. In U.S. Pat. No. 4,073,282, entitled "Radiation-Sensitive Record With Protected Sensitive Surface", issued Feb. 14, 1978, to Balas, Jr., et al, and assigned to U.S. Philips Corp., a transparent protective cover is disclosed for keeping dust away from the active layer of the disk and for keeping ablated material from being deposited upon the objective lens, wherein the protective cover is bonded to the disk substrate by an inner and an outer spacer ring, as discussed above. Work in the field of improving disk performance is predicated upon this basic patent.
Another approach to this problem is taken in U.S. Pat. No. 4,353,767, entitled "Method of Manufacturing An Optical Recording Disk", issued Oct. 12, 1982 to Wilkinson, and assigned to Discovision Associates, wherein a disk assembly is disclosed comprising a pair of transparent disks, each having a thicker inner and outer edge, such that when the thick edges are placed together, an interior chamber is formed. In this patent, either one or both of the inner disk surfaces can be coated with the active media layer, thus having the "spacer" rings manufactured as an integral part of the disk itself, eliminating the differential expansion problem. One problem with this patent is that should either of the protective surfaces be damaged, information recorded on the disk is effectively lost. An additional problem is the development of a transparent disk substrate which can meet the performance standards as found with aluminum substrates.
The present invention is an improvement over the prior art in that a mounting method is disclosed which will allow for the differential expansion of the disk substrate without warping of the protective cover. The disclosed invention is also an improvement in that the protective cover is removable thereby allowing a damaged window to be replaced or reused, as need be.
It is an object of the disclosed invention to provide a transparent protective cover for optical information recording disks.
It is another object of the disclosed invention to provide a transparent, low distortion, protective cover for optical information recording disks.
It is yet another object of the disclosed invention to provide a removable transparent protective cover for an optical information recording disks.
It is still another object of the disclosed invention to provide a reusable transparent protective cover for an optical information recording disks.
It is still yet a further object of the disclosed invention to provide an inexpensive means for mounting a transparent protective cover to optical information recording disks.